Add the login screen

[clenrock]

• Relevant issues: #26

• • *

What

With this pull request a working login screen is added to the app.

Why

In the future we want to secure the app with a password. It could also be that we use the password to encrypt the private key, but this still has to be discussed.

How

The login screen is the new first screen you see when you open the app. The password is now hardcoded and is 12345. If you write something else, it will give an error. If you leave it empty you can still continue, for development purposes.

[jverbraeken]

@clenrock Merge conflicts...